The present invention relates to collimators in radiation therapy devices, and more particularly to a technique for providing a high resolution multileaf collimator treatment delivery.
During conventional radiation therapy treatment, a beam of radiation, varying in angles and intensities, is directed at an area of a patient, e.g., at a tumor. Typical treatment field shapes, square, rectangular, or a modification thereof, result in a three-dimensional treatment volume that, unfortunately, may include healthy tissue and organs. For obvious safety reasons, the dose capable of being delivered to the tumor is limited by the healthy tissue and organs in the path of the radiation beam. Since cure rates for many tumors are a sensitive function of the dose they receive, reducing the amount of exposed healthy tissue and organs is highly desirable in order to be able to increase the dose delivered to the tumor. Methods of making the treatment volume correspond more closely with a tumor include moving solid-jaw blocks during treatment, scanning the radiation beam over the volume to be treated, and using a multileaf collimator to create an irregularly shaped field related to the shape of the tumor.
An example of a multileaf collimator arrangement positioned about the central axis of a radiation-emitting head for shaping an irradiated beam is disclosed in U.S. Pat. No. 5,166,531, issued to Hunzinger on Nov. 24, 1992. Two opposing arrays of side-by-side elongated radiation blocking collimator leaves act in place of opposing solid jaw blocks. Each leaf in each opposing array can be moved longitudinally towards or away from the central axis of the beam, thus defining a desired shape through which the radiation beam will pass.
An enhancement on the multileaf collimator approach is described in U.S. Pat. No. 5,591,983, issued to Yao on Jan. 7, 1997. In Yao, a multiple layer multileaf collimator design is formed through first and second layers of a plurality of elongated radiation blocking leaves. The leaves of each layer are arranged adjacent one another so as to form two opposed rows of adjacently positioned leaves and are movable in a longitudinal direction which can be either generally transverse to or in the same direction of the beam. The layers are arranged one above another in the beam direction and offset in a lateral direction, so that spaces between adjacent leaves of the first and second layers are positioned over and under, respectively, leaves of the respective first and second layers. The arrangement of the leaves allows a reduction in problems of radiation leakage between leaves of a multileaf collimator. However, the arrangement does not provide as fine a resolution as desired to allow more accurate block positioning for creating a block volume in correspondence with a tumor shape.
Multileaf collimators are being used to replace lead alloy blocks in many conformal treatments today. However, there are still a number of treatment cases that require the use of blocks because conformal shaping could not be adequately accomplished using an MLC. This is so because of the so-called xe2x80x9cstair-stepxe2x80x9d effect that occurs at field edges that are not perpendicular to the leaf face edges. An undulating dose pattern at the border of an irradiated volume results when the leaves are stepped to create an irregular shape. This distribution is unacceptable for field edges that are next to critical structures or when abutment of additional fields is planned.
There are solutions to address this problem of undulating dose patterns at a stepped MLC edge. First, blocks could continue to be used to define the shape. Second, the collimator could be rotated to bring the leaves in perpendicular to the field edge of interest. Finally, a Micro Multileaf Collimator could be utilized that has smaller leaves such as 0.5 cm widths.
Below are the advantages and disadvantages of each of these solutions.
1. Lead Alloy Blocks
Advantages:
Nicely defined edges around target and critical structures
Best penumbra results
Disadvantages:
All the same disadvantages as the block vs MLC argument
Block creating
Entering the room between each field
5% leakage
costly
2. Collimator Rotation
Advantages:
The collimator can be rotated to bring the leaves in perpendicular to the critical field edge
Disadvantages:
This technique is only viable if the MLC system is capable of rotating independently of any secondary jaw system, resulting in new mechanical complexity
This technique may move the undulating pattern to other positions along the field edge
3. Micro Multileaf Collimator
Advantages:
All the same advantages over a block solution
Better field edge definition than the original xe2x80x9cstair-steppedxe2x80x9d MLC shape
Disadvantages:
Most leakage of any solution
Increased number of leaves and motors can lead to serious mechanical and reliability issues
Design issues integrating it into the Collimator head
Can only handle smaller fields
Won""t have doubly focused leavesxe2x80x94poor penumbra
Manufacturability issues
Very costly
Accordingly, what is needed is a system and method for utilizing a multiple layer multileaf collimator arrangement that improves resolution and reduces leakage for radiation delivery. The present invention addresses such a need.
A method and system in accordance with the present invention uses the existing hardware and divides the treatment port dose into segments. In between each segment, the field, with respect to the beam, would be translated, and the leaf positions would be adjusted to maintain the tumor contour.
By integrating the above-identified methodology with a hardware system, accurate conformal radiation therapy is provided while minimizing leakage. In addition, through the present invention, higher dose rates can be provided while not appreciably affecting treatment time.
Accordingly, a system and method in accordance with the present invention provides for better defined edges around target and critical surfaces than a conventional multileaf collimator (i.e., effective penumbra). It provides for the same clearance as conventional MLC. It provides for the same maximum field size as a conventional MLC. Finally, a system and method in accordance with the present invention minimizes leakage between the leafs thereby minimizing leakage to the patient.